For three core high voltage cables, plastic fillers are traditionally introduced in between the cores during assembly. The fillers are designed to fit properly between the cores and to achieve an overall circular shape of the entire cable cross section. Normally, fillers are designed primarily to keep the power cable's roundness and for holding optical fibres.
An example of a filler is disclosed in SE 530 277 C2. This document discloses a power cable including three cores that are twinned with each other and arranged within a common external sheath. The power cable also comprises filler elements, each filler element being arranged in a respective space between the two cores and the external sheath. The filler elements have a profiled hollow body.
A power cable that is to be installed on the seabed is laid by special vessels that transport the power cable. The power cable may be wound on a large drum on the vessel during transport. During installation the power cable is wound off the drum, and the power cable may be fed onto a tower from which it is lowered into the sea. In order to be able to control the rate at which the power cable is lowered into the sea, tensioners on the tower are arranged to provide a suitable radial pressure to the power cable. The radial pressure applied to the power cable may be considerable, in order to hold the weight of a power cable extending down to the sea floor.
In case the power cable of SE 530 277 C2 would be installed in ultra deep-water, i.e. at depths starting at 1500 m, the filler walls may collapse during installation due to the large radial forces from tensioners. As a consequence, the optical fibre can be damaged and large local deformation may also occur on the cores due to excessive local contact forces with the filler ends. Irregular deformations on a core's insulation could affect the electrical field distribution around its cross section.